Electrolytic processing apparatus and electrolytic processing method

ABSTRACT

An electrolytic processing apparatus, prior to carrying out plating directly on, e.g., a ruthenium film of a substrate using the ruthenium film as a seed layer, can securely remove a passive layer formed on a surface of the ruthenium film even when the substrate is a large-sized high-resistance substrate, such as a 300-mm wafer, thereby reducing the terminal effect during the subsequent plating, improving the quality of a plated film and enabling filling of a void-free plated film into a fine interconnect pattern. The electrolytic processing apparatus includes: an anode disposed opposite a seed layer of a noble metal or a high-melting metal, formed on a substrate; a porous body impregnated with an electrolytic solution, disposed in a space, filled with the electrolytic solution, between the substrate and the anode; and a control section for controlling an electric field on a surface of the seed layer so that a reduction reaction takes place in the seed layer, thereby electrolytically and electrochemically removing a passive layer formed in the surface of the seed layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrolytic processing apparatus and an electrolytic processing method, and more particularly to an electrolytic processing apparatus and an electrolytic processing method which are useful for removing a passive layer from a surface of a ruthenium film, e.g., having a thickness of not more than 10 nm, formed on a surface of a substrate, such as a semiconductor wafer, prior to carrying out copper plating on the substrate surface using the ruthenium film as a seed layer to form LSI interconnects of copper on the surface of the ruthenium film.

2. Description of the Related Art

Copper plating is generally employed as a method for forming LSI interconnects when copper is used instead of aluminum as an interconnect material.

FIGS. 1A through 1C illustrate, in a sequence of process steps, an exemplary process for manufacturing a substrate having such copper interconnects. First, as shown in FIG. 1A, an insulating film 2, e.g., an SiO₂ oxide film or a film of low-k material, is deposited on a conductive layer 1 a, in which semiconductor elements has been formed, on a semiconductor base 1; and via holes 3 and trenches 4 as interconnect recesses are formed in the insulating film 2 by the lithography/etching technique. A barrier layer 5 is then formed on an entire surface of the substrate, and a seed layer 7, which serves as a feeding layer during electroplating, is formed on the barrier layer 5. A metal, such as tantalum, titanium, tungsten or ruthenium, or a nitride thereof, is generally used for the barrier layer 5.

Subsequently, as shown in FIG. 1B, copper plating is carried out onto a surface of the seed layer 7 of the substrate W to deposit a copper film 6 on the insulating film 2 while filling copper into the via holes 3 and the trenches 4. Thereafter, the copper film 6, the seed layer 7 and the barrier layer 5 on the insulating film 2 are removed by chemical mechanical polishing (CMP) to make a surface of the copper film 6, embedded in the via holes 3 and the trenches 4, approximately flush with the surface of the insulating film 2, thereby forming interconnects composed of the copper film 6 in the insulating film 2, as shown in FIG. 1C.

In conventional copper plating processes, a copper seed layer formed by, for example, sputtering, CVD, ALD or electroless plating is widely used as the seed layer 7. As interconnects are becoming increasingly finer, the copper seed layer 7 is becoming thinner year by year.

In particular, a thickness of a copper seed layer in the field region of a substrate is around 600 angstroms in the manufacturing of the 65-nm generation of semiconductor devices. A thickness of a copper seed layer is expected to be not more than 500 angstroms in the 45-nm generation of semiconductor devices, and not more than 300 angstroms in the 32-nm and the subsequent generations of semiconductor devices. The side coverage of a copper seed layer, as formed by the most-prevalent sputtering method, is generally 10 to 15%. Therefore, a copper seed layer used in the manufacturing of the 32-nm and the subsequent generations of semiconductor devices will have a very small thickness on the order of tens of angstroms in its portions lying on the sidewalls of via holes or trenches. The continuity of a seed layer will therefore be lost and the function of a seed layer will be insufficient, leading to significantly poor filling of copper into the recesses. There is therefore a movement to use, instead of sputtering, a more conformal film-forming method, such as CVD or ALD, to form a copper seed layer.

On the other hand, there is an attempt to eliminate a copper seed layer and carry out copper plating directly on a surface of a barrier layer, e.g., composed of a ruthenium film, using the barrier layer as a seed layer (feeding layer) during electroplating. This is partly because of instability of a copper material in an atmospheric environment. Thus, copper is easily oxidized in the air, forming a natural oxide film (copper oxide), having a thickness of a few angstroms to a few tens of angstroms, on a surface of a copper seed layer. Copper oxide is not electrically conductive and is easily soluble in an acidic plating solution.

When copper plating is carried out directly on a surface of a barrier layer, e.g., composed of a ruthenium film, using the barrier layer as a seed layer, there is a case, depending on the material of the barrier layer, in which a copper plated film with good morphology as formed by copper electroplating on a copper seed layer cannot be obtained, or a case in which the plated copper film has poor adhesion to the barrier layer. Further, in next-generation semiconductor devices, a thickness of a barrier layer will become tens of angstroms and the sheet resistance of a barrier layer will become tens of Q/sq to hundreds of Q/sq. Therefore, compared to the case of carrying out plating on a surface of a copper seed layer, the problem of terminal effect will become more serious when carrying out plating directly on a surface of a barrier layer using the barrier layer as a seed layer.

A technique for direct plating on a barrier layer has been proposed which involves adjusting deposition potentials of barrier layer/copper and copper/copper using a copper sulfate plating solution containing additives, and gradually increasing the electric current applied, thereby filling copper into interconnect recesses covered with a barrier layer (see US Patent Publication No. 2004/0069648 and U.S. Pat. No. 6,974,531). Though this technique enables uniform filling of copper into interconnect recesses covered with a barrier layer, however, a global thickness of a copper plated film, formed on a substrate, differs between the center portion and the edge portion of the film especially when the substrate is a large 300-mm wafer (substrate), which may cause a problem in a later CMP process which is generally carried out after plating.

When there is a passive layer (ruthenium oxide) formed on a surface of a ruthenium film as a barrier layer, copper will be deposited in a particulate form upon direct copper plating on the surface of the ruthenium film (barrier layer) using the ruthenium film as a seed layer, which can cause voids in copper embedded in a fine interconnect pattern and surface roughness of the plated film on the substrate. There is a report that to solve such problems, it is effective to carry out pretreatment (electrolytic processing), prior to copper plating, by using a mixed solution of 1.8 mol/L (17.6 wt %) of sulfuric acid and 1 mmol/L of NaCl as a pretreatment solution (electrolytic solution) and applying a voltage with a ruthenium film as a cathode (see T. P. Moffat et al., “Electrodeposition of Cu on Ru Barrier Layers for Damascene Processing”, journal of the Electrochemical Society, 153(1) C37-C50 (2006)). This pretreatment (electrolytic processing), because of sodium contained in the pretreatment solution (electrolytic solution), however, is generally difficult to use in a semiconductor manufacturing process.

Further, it has been proposed to remove an oxide film formed on a surface of a metal film, such as a ruthenium film, by carrying out a cathodic treatment (pretreatment) at a voltage, e.g., in the range of about 0V to about −0.5V or at a current density, e.g., in the range of 0.05 mA/cm² to about 1 mA/cm² (see Published Japanese Translation of International Patent Publication No. 2008-502806). However, it is considered that when a high-resistance substrate having a surface ruthenium film, e.g., with a thickness of not more than 10 nm, especially a large-sized one such as a 300-mm wafer, is subjected to the cathodic treatment (pretreatment) using such a voltage or current density, because of the terminal effect, it will be difficult to uniformly process the entire substrate surface.

The applicant has proposed a method in which a passive layer formed on a surface of a ruthenium film is electrochemically removed by electrolytic processing using an electrolytic solution having an electric conductivity of not more than 0.4/Ω·cm, such as sulfuric acid having a concentration of not more than 10 wt % (see Japanese Patent laid-Open Publication No. 2008-98449). However, it has turned out that though this method is effective for a relatively small-sized substrate such as a test chip, in the case of a large-sized high-resistance substrate, such as a 300-mm wafer, having a surface ruthenium film, e.g., with a thickness of not more than 10 nm, due to the terminal effect, it is difficult to uniformly process the entire substrate surface.

SUMMARY OF THE INVENTION

When carrying out copper plating directly on a surface of a barrier layer using the barrier layer as a seed layer (feeding layer), especially when carrying out copper electroplating directly on a surface of a ruthenium film having a thickness of not more than 10 nm, which is expected to be used in the 32-nm generation of semiconductor devices, using the ruthenium film as a seed layer, it is necessary to remove a passive layer (ruthenium oxide), formed on a surface of the ruthenium film, prior to the copper plating. Especially, with the progress toward a thinner ruthenium film, the proportion of a passive layer (ruthenium oxide) in a ruthenium film becomes larger. Therefore, the necessity for removal of a passive layer (ruthenium oxide) from a surface of a ruthenium film will become higher.

This is because a passive layer (ruthenium oxide) has a very high electric resistance. If copper plating is carried out directly on a ruthenium film, which itself has a high resistance, with a passive layer formed on a surface, because of increased terminal effect due to the passive layer, it is difficult to form a copper plated film with high in-plane uniformity of a film thickness especially on a large-sized high-resistance substrate, such as a 300-mm wafer. In addition, plated copper will deposit in a particulate form on the surface of the passive layer (ruthenium oxide), which may cause the formation of voids in copper embedded in a fine interconnect pattern and also cause surface roughness of a plated film formed on the substrate surface.

The present invention has been made in view of the above situation. It is therefore an object of the present invention to provide an electrolytic processing apparatus and an electrolytic processing method which, prior to carrying out plating directly on, e.g., a ruthenium film of a substrate using the ruthenium film as a seed layer, can securely remove a passive layer (ruthenium oxide) formed on a surface of the ruthenium film even when the substrate is a large-sized high-resistance substrate, such as a 300-mm wafer, thereby reducing the terminal effect during the subsequent plating, improving the quality of a plated film and enabling filling of a void-free plated film into a fine interconnect pattern.

In order to achieve the object, the present invention provides an electrolytic processing apparatus comprising: an anode disposed opposite a seed layer of a noble metal or a high-melting metal, formed on a substrate; a porous body impregnated with an electrolytic solution, disposed in a space, filled with the electrolytic solution, between the substrate and the anode; and a control section for controlling an electric field on a surface of the seed layer so that a reduction reaction takes place in the seed layer, thereby electrolytically and electrochemically removing a passive layer formed on the surface of the seed layer.

The resistance between the substrate and the anode can be increased by disposing the porous body impregnated with an electrolytic solution in a space, filled with the electrolytic solution, between the substrate and the anode. This can reduce the terminal effect during the electrolytic processing. In addition, in the electrolytic processing to electrochemically remove a passive layer formed on a surface of a seed layer, the voltage or electric current applied between the anode and the seed layer may be gradually increased. This can remove the passive layer on the surface of the seed layer gradually from the periphery toward the center of the substrate while reducing the terminal effect, thus enabling uniform electrolytic processing of the entire surface of the substrate even when the substrate is a large-sized high-resistance substrate, such as a 300-mm wafer.

Preferably, a shield plate for limiting the area of electrolytic processing is disposed between the porous body and the anode.

By limiting the electrolytic processing area of a substrate by the shield plate, it becomes possible to lower the maximum applied voltage or the maximum applied current necessary for the cathodic processing of the entire substrate from the peripheral portion to the central portion. The shield plate preferably has a diaphragm mechanism capable of adjusting the aperture area corresponding to the electrolytic processing area.

The anode is comprised, for example, of a disk-shaped anode, or one or more ring-shaped anodes, or a combination thereof.

For example, the anode may be comprised of a combination of a disk-shaped divided anode and a plurality of ring-shaped divided anodes. By independently controlling the respective divided anodes, it becomes possible to easily control the electric field on a surface of a seed layer, enabling more uniform processing of the entire substrate.

Preferably, the electrolytic processing apparatus further comprises a gas removal mechanism for removing a gas generated from the surface of the seed layer during the electrolytic processing.

The gas removal means is, for example, means for circulating the cathode-side electrolytic solution, means for rotating the substrate during electrolytic processing or means for vertically moving or tilting a head holding the anode.

The seed layer is, for example, comprised of a ruthenium film or a ruthenium alloy film having a thickness of not more than 10 nm.

The present invention also provides an electrolytic processing method comprising: disposing a porous body between a substrate having a seed layer of a noble metal or a high-melting metal and an anode disposed opposite the seed layer; filling an electrolytic solution into a space between the substrate and the anode while impregnating the porous body with the electrolytic solution; and electrolytically and electrochemically removing a passive layer formed on a surface of the seed layer while controlling an electric field on the surface of the seed layer so that a reduction reaction takes place in the seed layer.

Preferably, the voltage or electric current applied between the seed layer and the anode is gradually increased in accordance with the electrolytic processing area of the substrate.

A shield plate may be disposed between the anode and the porous body to limit the area of electrolytic processing.

The present invention provides another electrolytic processing method comprising: filling an electrolytic solution containing sulfuric acid as an electrolyte into a space between a substrate, having a seed layer of a noble metal or a high-melting metal, and an anode disposed opposite the seed layer; and applying an electric current between the seed layer and the anode in such a manner as to satisfy the following formula (1), thereby electrolytically and electrochemically removing a passive layer formed on a surface of the seed layer:

A≧1.15×B+5  (1)

wherein “A” represents the sulfuric acid concentration of the electrolytic solution (g/L), and “B” represents current density (mA/cm²).

According to the present invention, a passive layer (ruthenium oxide) formed on a surface of a ruthenium film can be electrochemically removed over an entire surface of a substrate even when the substrate is a large-sized high-resistance substrate, such as a 300-mm wafer, having the ruthenium film with a thickness of not more than 10 nm, and subsequently copper electroplating can be carried out directly on the surface of the ruthenium film using the ruthenium film as a seed layer. This makes it possible to reduce the terminal effect during plating, speed up plating and shorten time taken for plating to reach the center of a substrate and, in addition, form a fine metal plated film, such as a copper, with a uniform thickness over the entire substrate surface while filling the plating metal into a fine interconnect pattern without forming voids in the embedded metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C are diagrams illustrating, in a sequence of process steps, a conventional process for the formation of copper interconnects;

FIG. 2 is a layout plan view of a substrate processing apparatus incorporating an electrolytic processing apparatus according to an embodiment of the present invention;

FIG. 3 is a schematic cross-sectional view of the electrolytic processing apparatus shown in FIG. 2;

FIG. 4 is a graph showing the relationship between the sulfuric acid concentration of an electrolytic solution for use in the present invention and current density between an insoluble anode and a ruthenium film;

FIG. 5 is a flow chart of a process as carried out in the substrate processing apparatus shown in FIG. 2;

FIGS. 6A and 6B are diagrams showing the morphology of a copper plated film as formed by first carrying out electrolytic processing to electrochemically remove a passive layer formed on a surface of a ruthenium film, and subsequently carrying out copper electroplating on the ruthenium film using the ruthenium film as a seed layer;

FIG. 7 is a graph showing the bottom-up performance of plating, determined by chip test for samples as prepared by first carrying out electrolytic processing of an interconnect substrate having a ruthenium film with varying current densities and varying sulfuric acid concentrations, using the electrolytic processing apparatus shown in FIG. 3, to electrochemically remove a passive layer formed on the surface of the ruthenium film, and subsequently carrying out copper plating on the ruthenium film using the ruthenium film as a seed layer;

FIG. 8 is a diagram explaining the bottom-up performance;

FIG. 9 is a schematic view of the main portion of an electrolytic processing apparatus according to another embodiment of the present invention;

FIG. 10 is a graph showing change with time in current value during electrolytic processing in the electrolytic processing apparatus shown in FIG. 9; and

FIG. 11 is a schematic diagram showing another insoluble anode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described with reference to the drawings. The following description illustrates an exemplary case in which a substrate W, having a ruthenium film as the barrier layer 5 shown in FIG. 1A, is prepared, and a copper film 6 (see FIG. 1B) is formed directly on a surface of the ruthenium film (barrier layer 5), without forming the seed layer 7 shown in FIG. 1A, by carrying out copper electroplating using the ruthenium film as a seed layer. The present invention may also be applied to a case in which instead of a ruthenium film, a ruthenium alloy film or a film of another noble metal or high-melting metal is used as a seed layer during electroplating, and a plated film is formed directly on a surface of the seed layer.

FIG. 2 shows a layout plan of a substrate processing apparatus incorporating an electrolytic processing apparatus according to an embodiment of the present invention. As shown in FIG. 2, the substrate processing apparatus includes a rectangular apparatus frame 12 having a control panel 10. In the apparatus frame 12 are disposed two loading/unloading sections 14 for carrying in a substrate cassette in which a plurality of substrates are housed, two bevel etching/back surface cleaning apparatuses 16, a substrate station 18, a rinsing/drying apparatus 20, one electrolytic processing apparatus 22 and four copper electroplating apparatuses 24. A first transport robot 26 is movably disposed between the loading/unloading sections 14, the bevel etching/back surface cleaning apparatuses 16, the substrate station 18 and the rinsing/drying apparatus 20, and a second transport robot 28 is movably disposed between the substrate station 18, the rinsing/drying apparatus 20, the electrolytic processing apparatus 22 and the copper electroplating apparatuses 24.

FIG. 3 shows a schematic view of the electrolytic processing apparatus 22 shown in FIG. 2. The electrolytic processing apparatus 22 is to electrolytically and electrochemically remove, prior to copper electroplating, a passive film (ruthenium oxide) formed on a surface of a ruthenium film which serves as a seed layer during copper electroplating, and is comprised mainly of a substrate holding section 30 and an anode head 32. The substrate holding section 30 includes a substrate stage 34 for holding, e.g., by attraction, a substrate W with its front surface (with ruthenium film formed) facing upwardly. The substrate stage 34 is coupled to an upper end of a rotating shaft 36 extending from a rotation mechanism (not shown). By thus connecting the substrate stage 34 to the upper end of the rotating shaft 36 and rotating the substrate W by rotating the rotating shaft 36, hydrogen gas generated during electrolytic processing of a ruthenium film is removed from a surface of the ruthenium film. A gas removal mechanism is thus constructed.

The substrate holding section 30 has a ring-shaped seal ring 38 which is located above the substrate stage 34 and which, when the substrate W held by the substrate stage 34 is raised, comes into pressure contact with a peripheral portion of the upper surface of the substrate W to seal the peripheral portion, and cathode contacts 40 for contact with the ruthenium film at a peripheral portion of the upper surface of the substrate W and at an outer position than the contact portion of the substrate surface with the seal ring 38, to make the ruthenium film serve as a cathode. With this structure, when the peripheral portion of the upper surface of the substrate W is sealed by the seal ring 38, a substrate-side electrolytic solution chamber, circumferentially defined by the seal ring 38, is formed over the upper surface of the substrate W. Because the cathode contacts 40 are located outside the seal ring 38, contact of the cathode contacts 40 with an electrolytic solution 62 in the substrate-side electrolytic solution chamber can be avoided.

In this embodiment, the seal ring 38 and the cathode contacts 40, while keeping contact with the peripheral portions of the upper surface of the substrate W held by the substrate stage 34, are configured to rotate together with the substrate stage 34.

The anode head 32 includes a vertically-movable lifting shaft 42, a downwardly-open, bottomed cylindrical housing 44 coupled to a lower end of the lifting shaft 42, and a porous body 46 disposed such that it closes the lower-end opening of the housing 44. The housing 44, in its inner circumferential surface, has a recessed portion 44 a, while the porous body 46 has at its top a flange portion 46 a. The porous body 46 is held in the housing 44 with the flange portion 46 a inserted into the recessed portion 44 a.

A shield ring 48, for preventing an electric current from flowing out of the circumferential surface of the porous body 46, is wound around the circumferential surface of the porous body 46. The shield ring 48 may be made of a flexible material, such as fluoro rubber. Located in the recessed portion 44 a of the housing 44, an O-ring 50 for preventing leakage of the electrolytic solution is interposed between the flange portion 46 a of the porous body 46 and the housing 44.

The porous body 46 is, for example, composed of a porous ceramic, such as alumina, SiC, mullite, zirconia, titania or cordierite, or a hard porous body, such as a sintered body of polypropylene or polyethylene, or a composite thereof. The porosity of the porous body 46 is, for example, not more than 19%. Though the porous body 46 itself is an insulating material, it has an electric conductivity when it contains an electrolytic solution therein. In particular, the electrolytic solution penetrates into the porous body 46 in the thickness direction through complicated, fairly long paths of the pores. This can provide the porous body 46 containing the electrolytic solution with an electric conductivity which is lower than the electric conductivity of the electrolytic solution.

The provision of the porous body 46, which thus has a high electric resistance, at the opening of the housing 44 can make the influence of the resistance of the ruthenium film as small as negligible upon electrolytic processing of the ruthenium film. Thus, a difference in current density in the surface of the substrate W due to the electric resistance of the substrate surface can be made small, thereby enhancing the in-plane uniformity of electrolytic processing.

The use of the porous body 46, e.g., a porous ceramic having a porosity of not more than 19%, can separate the electrolytic solution on the substrate W side from the electrolytic solution on the insoluble anode 52 side by the porous body 46. This makes it possible to replace only the electrolytic solution in the substrate-side electrolytic solution chamber, formed over the surface of the substrate W and surrounded by the seal ring 38, with a plating solution and carry out plating after carrying out electrolytic processing with the electrolytic solution in the substrate-side electrolytic solution chamber. This method thus enables electrolytic processing and plating to be carried out successively in the single electrolytic cell simply by replacement of processing solutions.

In the housing 44, an insoluble anode 52 having a large number of through-holes therein is disposed above the porous body 46 such that it faces the ruthenium film of the substrate W held by the substrate stage 34. The insoluble anode 52, for example composed of a titanium base and an iridium oxide coating, does not dissolve in an electrolytic solution during electrolytic processing, and therefore is not subject to deformation. The use of the insoluble anode 52 can therefore render its replacement unnecessary. In the case where the electrolytic solution is separated into the cathode-side solution and the substrate (anode)-side solution by an ion exchanger or the like, a soluble anode may be used instead of the insoluble anode.

The insoluble anode 52 is electrically connected to an anode conducting wire 56 a extending from the anode of a power source 54, while the cathode contacts 40 are electrically connected to a cathode conducing wire 56 b extending from the cathode of the power source 54. The power source 54 is connected to a control section 58 which controls an electric current or a voltage applied from the power source 54 to between the insoluble anode 52 and the ruthenium film which serves as a cathode when in contact with the cathode contacts 40.

To the housing 44 is connected an oxygen gas discharge pipe 60 for discharging oxygen, which has accumulated over the electrolytic solution 62 in the housing 44, to the outside. Further, though not shown diagrammatically, an electrolytic solution supply pipe for supplying the electrolytic solution 62 into the housing 44 and an electrolytic solution discharge pipe for discharging the electrolytic solution 62 in the housing 44 to the outside are connected to the housing 44. Further, either lateral to the housing 44 or in the interior of the sidewall of the housing 44 are provided an electrolytic solution supply section for supplying the electrolytic solution 62 into the space over the surface of the substrate W and surrounded by the seal ring 38 (substrate-side electrolytic solution chamber) and an electrolytic solution discharge section for discharging the electrolytic solution 62 in the substrate-side electrolytic solution chamber to the outside. In this embodiment, sulfuric acid having a concentration of not more than 100 g/L, e.g., 80 g/L, is used as the electrolytic solution 62.

FIG. 4 shows the relationship between the sulfuric acid concentration of the electrolytic solution 62 and current density between the insoluble anode 52 and the ruthenium film in contact with the cathode contacts 40 and serving as a cathode. In this embodiment, an electric current is applied between the insoluble anode 52 and the ruthenium film, in contact with the cathode contacts 40 and serving as a cathode, in such a manner as to satisfy the following formula (1), thereby electrolytically and electrochemically removing a passive layer (ruthenium oxide) formed on the surface of the ruthenium film:

A≧1.15×B+5  (1)

wherein “A” represents the sulfuric acid concentration of the electrolytic solution 62 (g/L), and “B” represents current density (mA/cm²).

The operation of the electrolytic processing apparatus 22 will now be described. First, the substrate W is held with its front surface (with the ruthenium formed) facing upwardly by the substrate stage 34 which attracts thereto the lower surface of the substrate W. At this moment, the substrate W is in a lowered position. The substrate stage 34 is then raised to bring the peripheral portion of the upper surface of the substrate W, held by the substrate stage 34, into pressure contact with the seal ring 38, thereby forming the substrate-side electrolytic solution chamber, circumferentially defined by the seal ring 38, over the upper surface of the substrate W. At the same time, the ruthenium film, at the peripheral portion of the upper surface of the substrate W and at the outer position than the seal ring 38, is brought into contact with the cathode contacts 40.

Next, the anode head 32 in a raised position is lowered, and the lowering of the anode head 32 is stopped when the porous body 46 has reached a position not into contact with but close to the upper surface of the substrate W. The narrower the gap “G” between the porous body 46 and the substrate W held by the substrate stage 34 is, the larger is the terminal effect reducing effect of the porous body 46. Therefore, the gap “G” is preferably not more than 10 mm, more preferably not more than 0.5 mm. In the anode head 32, the electrolytic solution 62 is supplied into the housing 44 while drawing the electrolytic solution 62 from the housing 44, thereby impregnating the porous body 46 with the electrolytic solution 62.

The electrolytic solution 62 is injected into the region (substrate-side electrolytic solution chamber) surrounded by the seal ring 38 to fill the space between the substrate W and the porous body 46 with the electrolytic solution 62. If necessary, the electrolytic solution 62 is withdrawn by suction form the substrate-side electrolytic solution chamber and returned to the chamber in a circulatory manner. In this state, while rotating the substrate stage 34, thereby rotating the substrate W together with the seal ring 38 and the cathode contacts 40, the insoluble anode 52 is electrically connected to the anode of the power source 54 and the cathode contacts 40 to the cathode of the power source 54 to carry out electrolytic processing of the substrate surface, thereby electrolytically and electrochemically removing a passive film (ruthenium oxide) present on the surface of the ruthenium film serving as a cathode. Thus, in this embodiment, water is subjected to cathodic electrolysis with the electrolytic solution 62, consisting of sulfuric acid having a concentration of not more than 100 g/L, to generate hydrogen. The hydrogen electrochemically removes the passive film (ruthenium oxide) present on the surface of the ruthenium film. The hydrogen generated at this time is removed from the surface of the ruthenium film by the rotation of the substrate W.

During the electrolytic processing, the voltage or electric current applied between the insoluble anode 52 and the ruthenium film serving as a cathode is gradually increased from a low value in accordance with the electrolytic processing area by the control section 58 so that the passive layer on the surface of the ruthenium film is removed gradually from the periphery toward the center of the substrate W.

As described above, the resistance between the substrate W and the insoluble anode 52 is increased by disposing the porous body 46 impregnated with the electrolytic solution in the space, filled with the electrolytic solution 62, between the substrate Wand the insoluble anode 52. This can reduce the terminal effect upon the electrolytic processing. In addition, in the electrolytic processing to electrochemically remove the passive layer formed on the surface of the ruthenium film, the voltage or electric current applied between the insoluble anode 52 and the ruthenium film is gradually increased. This can remove the passive layer on the surface of the ruthenium film gradually from the periphery toward the center of the substrate while reducing the terminal effect, thus enabling uniform electrolytic processing of the entire surface of the substrate even when the substrate is a large-sized high-resistance substrate, such as a 300-mm wafer.

After completion of the electrolytic processing, the insoluble anode 52 and the cathode contacts 40 are disconnected from the power source 54, and the rotation of the substrate stage 34 is stopped. After raising the anode head 32, the electrolytic solution 62 remaining on the upper surface of the substrate W is removed and recovered, e.g., by suction, and the substrate W after electrolytic processing is transported for the next process step.

The operation of the substrate processing apparatus shown in FIG. 2 will now be described with reference to FIG. 5. First, a substrate cassette, in which a plurality of substrates W are housed, is carried into the loading/unloading section 14 in the apparatus frame 12. The first transport robot 26 takes one substrate W out of the substrate cassette carried into the loading/unloading section 14 and transports the substrate W to the substrate station 18. The second transport robot 28 receives the substrate W from the substrate station 18 and transfers the substrate W to the substrate stage 34 of the electrolytic processing apparatus 22.

After receiving the substrate by the substrate stage 34, the electrolytic processing apparatus 22 carries out electrolytic processing of the substrate W, held by the substrate stage 34, in the above-described manner to electrochemically remove a passive film (ruthenium oxide) present on the surface of the ruthenium film. If the electrolytic processing apparatus 22 has a function to rinse with pure water a surface of a substrate after electrolytic processing and dry the substrate by rotating it at a high speed, rinsing and drying of the substrate W is carried out in the electrolytic processing apparatus 22. Otherwise the substrate W after electrolytic processing is transported by the second transport robot 28 to the rinsing/drying apparatus 20, where the substrate is rinsed and dried. It is possible, in some cases, to omit drying or both rinsing and drying.

The second transport robot 28 receives the substrate from the electrolytic processing apparatus 22 or from the rinsing/drying apparatus 20, and transports the substrate to the substrate stage of the copper electroplating apparatus 24. After receiving the substrate by the substrate stage, the copper electroplating apparatus 24 carries out copper electroplating of the substrate using the ruthenium film as a seed layer to form a copper plated film on the surface of the ruthenium film. The substrate after plating is transported by the second transport robot 28 to the rinsing/drying apparatus 20, where the substrate is rinsed and dried. If the copper electroplating apparatus 24 has a function to rinse with pure water a surface of a substrate after plating and dry the substrate by rotating it at a high speed, rinsing and drying of the substrate W may be carried out in the copper electroplating apparatus 24.

The first transport robot 26 receives the dried substrate from the rinsing/drying apparatus 20 and transfers the substrate to the bevel etching/back surface cleaning apparatus 16. The bevel etching/back surface cleaning apparatus 16 carries out bevel etching to etch off a copper plated film, etc. adhering to the bevel portion of the substrate, and cleaning of the back surface of the substrate. The first transport robot 26 receives the substrate from the bevel etching/back surface cleaning apparatus 16 and returns the substrate to the substrate cassette in the loading/unloading section 14.

The sequence of substrate processing steps is thus completed.

The above-described substrate processing process can carry out, in a successive one-by-one manner, copper electroplating of a substrate after carrying out electrolytic processing of the substrate to electrochemically remove a passive layer (ruthenium oxide) formed on a surface of a ruthenium film, e.g., having a thickness of not more than 10 nm and having a high sheet resistance. This can prevent an oxide film (ruthenium oxide) from growing on the surface of the ruthenium film during the period after the removal of the passive layer until the initiation of copper plating. Furthermore, it becomes possible to control the time period after the electrolytic processing until the initiation of copper electroplating at a constant time.

FIGS. 6A and 6B show the morphology of a copper plated film as formed by first carrying out electrolytic processing to electrochemically remove a passive layer (ruthenium oxide) formed on a surface of a ruthenium film, and subsequently carrying out copper electroplating on the ruthenium film using the ruthenium film as a seed layer. As can be seen from FIGS. 6A and 6B, compared to the case of forming a copper plated film on a ruthenium film without electrochemically removing a passive layer from the surface of the ruthenium film, a fine particulate copper plated film can be formed uniformly on a substrate surface by electrochemically removing a passive layer (ruthenium oxide) from the surface of the ruthenium film in advance.

FIG. 7 shows the bottom-up performance of a plating, determined by chip test for samples as prepared by first carrying out electrolytic processing of an interconnect substrate having a ruthenium film, having a thickness of 2 nm and a sheet resistance of 150 Ω/sq, with varying current densities and varying sulfuric acid concentrations, using the electrolytic processing apparatus shown in FIG. 3, to electrochemically remove a passive layer formed on a surface of the ruthenium film, and subsequently carrying out copper plating on the ruthenium film using the ruthenium film as a seed layer. As shown in FIG. 8, the bottom-up performance is determined by the ratio of the thickness “b” of a plated film in interconnects of a substrate to the thickness “a” of the plated film in the field area of the substrate (b/a). In the estimation of interconnect plating, a higher ratio b/a indicates a better bottom-up performance. All the samples were subjected to plating, carried out under the same conditions, within 10 minutes after electrolytic processing.

The following six combinations of sulfuric acid concentration and current density were used in the electrolytic processing:

(1) Sulfuric acid conc. 0.8 (g/L), current density 40 (mA/cm²) (2) Sulfuric acid conc. 8 (g/L), current density 40 (mA/cm²) (3) Sulfuric acid conc. 8 (g/L), current density 80 (mA/cm²) (4) Sulfuric acid conc. 80 (g/L), current density 40 (mA/cm²) (5) Sulfuric acid conc. 80 (g/L), current density 80 (mA/cm²) (6) Sulfuric acid conc. 80 (g/L), current density 120 (mA/cm²)

FIG. 7 depicts a straight line corresponding to the equality in the above formula (1) (A=1.15×B+5). As can be seen from FIG. 7, the bottom-up performance is poor when the combinations of low current density and high sulfuric acid concentration, which fall within the range that does not satisfy the above formula (1) (A<1.15×B+5), are used in the electrolytic processing, indicating poor removal of the passive layer from the ruthenium film by electrolytic processing. It is inferred from the data that a passive layer will not be adequately removed by electrolytic processing using a sulfuric acid concentration in the range of 10 to 100 g/L and a current density in the range of 0.05 to 1 mA/cm², and that the bottom-up performance will be poor in the subsequent plating.

Interconnect substrate samples having a ruthenium film, having a thickness of 2 nm and a sheet resistance of 150 Ω/sq, were subjected to electrolytic processing at a sulfuric acid concentration of 80 g/L and a current density of 40 mA/cm² using a porous body having a porosity of 19%. The respective substrate samples were then subjected to copper plating under the same conditions within 10 minutes after the electrolytic processing to evaluate the bottom-up performance. As a result, void-free copper was filled into an interconnect pattern with good bottom-up performance in a 20-mm square test chip sample. On the other hand, a difference in the bottom-up performance was observed in a 300-mm wafer substrate: the same bottom-up performance as the test chip sample was observed in the edge portion of the substrate, whereas the bottom-up performance was found to be poor in the center portion of the substrate. This indicates that the electrolytic processing was not uniformly effected over the entire substrate surface, i.e., from the edge to the center of the substrate. In this regard, when the same 300-mm wafer substrate, having the same ruthenium film (having a thickness of 2 nm and a sheet resistance of 150 Ω/sq) but whose passive layer had been confirmed to be removed from the entire surface, was subjected to copper plating under the same conditions, void-free copper was filled into an interconnect pattern with good bottom-up performance in the entire surface from the edge to the center of the substrate.

In order to carry out electrolytic processing uniformly from the edge to the center of a substrate, it is conceivable to lengthen the processing time or increase the current density. A long processing time or a high current density, however, may lead to generation of a considerable amount of gas, which can cause the formation of voids or cause damage to an edge portion of a substrate. For a high-resistance substrate, the use of a porous body alone is not enough to reduce the terminal effect, and thus is incapable of uniformly processing the entire substrate. The entire substrate can be processed uniformly by controlling the electric field on the substrate in addition to the use of a porous body.

For example, a low voltage or current is first applied between an anode and a ruthenium film serving as a cathode to remove a passive layer only in a peripheral portion of a substrate, and removal of the passive layer is allowed to proceed to the center of the substrate while increasing the voltage or current applied between the ruthenium film and the anode with the progress of removal of the passive layer. By thus removing the passive layer gradually from the periphery to the center of the substrate, the substrate resistance can be lowered, whereby the terminal effect can be reduced. In addition, the maximum applied voltage or the maximum applied current for electrolytic processing to the center of the substrate can be lowered.

FIG. 9 shows the main portion of an electrolytic processing apparatus according to another embodiment of the present invention. This embodiment differs from the preceding embodiment in that a shield plate 70, having a diaphragm mechanism such as an iris diaphragm, for limiting the area of electrolytic processing is disposed between the porous body 46 and the insoluble anode 52. The closer the shield plate 70 is to the porous body 46, the larger is the effect of the shield plate 70. The distance between the porous body 46 and the shield plate 70 is therefore preferably not more than 10 mm, more preferably not more than 0.5 mm. The shield plate 70 may be disposed in contact with the porous body 46.

In this embodiment, the shield plate 70 has a diaphragm mechanism which opens from the center toward the periphery. When a diaphragm mechanism, which opens from the center toward the periphery, is thus used, a large amount of electric current flows from the central portion of a substrate, lying right under the aperture of the shield plate 70. Therefore, the applied current is increased in accordance with the aperture area as shown by the curve (A) in FIG. 10. The shield plate 70 may have a diaphragm mechanism which opens from the periphery toward the center. In this case, a large amount of electric current flows from the peripheral portion of a substrate, and the area in which electric current flows decreases as the diaphragm mechanism closes. Therefore, the applied current is decreased in accordance with the aperture area as shown by the curve (B) in FIG. 10. By thus controlling the aperture of the shield plate 70 and the current value, electrolytic processing of an entire surface of a substrate becomes possible.

It is also possible to install a hollow disk-shaped shield plate between the insoluble plate and the porous body in order to prevent concentration of electric current in a peripheral portion of a substrate W near the cathode contacts 40. In this case, it becomes possible to control the area of highest electric field by the inner diameter of the shield plate, thereby lowering the maximum applied voltage or the maximum applied current necessary for electrolytic processing of the entire substrate surface from the edge to the center of the substrate. Further, it is possible to interpose a shield plate, having holes whose density increases toward the center, between the insoluble anode and the porous body to control the electric field distribution on an entire surface of a substrate.

FIG. 11 shows another example of an insoluble anode 52 for use in the present invention. The insoluble anode 52 is comprised of a disk-shaped first divided anode 52 a centrally located, a ring-shaped second divided anode 52 b surrounding the circumference of the first divided anode 52 a, and a ring-shaped third divided anode 52 c surrounding the circumference of the second divided anode 52 b. The divided anodes 52 a, 52 b and 52 c are provided with power sources 54 a, 54 b and 54 c, respectively. The power sources 54 a, 54 b, 54 c are independently controlled by the control section 58.

When an insoluble anode comprising a combination of divided anodes is used as in this embodiment, a higher voltage or electric current may be applied to a divided anode more centrally located than peripherally located. Thus, in this embodiment, the applied voltage or electric current may be decreased in the order of the first divided anode 52 a, the second divided anode 52 b and the third divided anode 52 c. This can reduce the terminal effect on a substrate, enabling uniform electrolytic processing of an entire substrate surface.

When the shield plate 70 or the divided anodes 52 a, 52 b, 52 c is used, as described above, the porous body 46 not only has the effect of reducing the terminal effect of a substrate but has the effect of facilitating control of the electric field on the substrate by the shield plate 70 or the divided anodes 52 a, 52 b, 52 c as well. If the porous body 46 is not used, concentration of electric field, depending on the inner diameter of the shield plate or on the diameter of a divided anode, will occur, which will make control of the electric field on a substrate difficult.

While the present invention has been described with reference to the embodiments thereof, it will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described above, but it is intended to cover modifications within the inventive concept. For example, though the use of copper as an interconnect material has been described, a copper alloy may be used instead of copper. 

1. An electrolytic processing apparatus comprising: an anode disposed opposite a seed layer of a noble metal or a high-melting metal, formed on a substrate; a porous body impregnated with an electrolytic solution, disposed in a space, filled with the electrolytic solution, between the substrate and the anode; and a control section for controlling an electric field on a surface of the seed layer so that a reduction reaction takes place in the seed layer, thereby electrolytically and electrochemically removing a passive layer formed on the surface of the seed layer.
 2. The electrolytic processing apparatus according to claim 1, wherein a shield plate for limiting the area of electrolytic processing is disposed between the porous body and the anode.
 3. The electrolytic processing apparatus according to claim 1, wherein the anode is comprised of a disk-shaped anode, or one or more ring-shaped anodes, or a combination thereof.
 4. The electrolytic processing apparatus according to claim 1 further comprising a gas removal mechanism for removing a gas generated from the surface of the seed layer during the electrolytic processing.
 5. The electrolytic processing apparatus according to claim 1, wherein the seed layer is comprised of a ruthenium film or a ruthenium alloy film having a thickness of not more than 10 nm.
 6. An electrolytic processing method comprising: disposing a porous body between a substrate having a seed layer of a noble metal or a high-melting metal and an anode disposed opposite the seed layer; filling an electrolytic solution into a space between the substrate and the anode while impregnating the porous body with the electrolytic solution; and electrolytically and electrochemically removing a passive layer formed on a surface of the seed layer while controlling an electric field on the surface of the seed layer so that a reduction reaction takes place in the seed layer.
 7. The electrolytic processing method according to claim 6, wherein the voltage or electric current applied between the seed layer and the anode is gradually increased in accordance with the electrolytic processing area of the substrate.
 8. The electrolytic processing method according to claim 6, wherein a shield plate is disposed between the anode and the porous body to limit the area of electrolytic processing.
 9. The electrolytic processing method according to claim 6, wherein the seed layer is comprised of a ruthenium film or a ruthenium alloy film having a thickness of not more than 10 nm.
 10. An electrolytic processing method comprising: filling an electrolytic solution containing sulfuric acid as an electrolyte into a space between a substrate, having a seed layer of a noble metal or a high-melting metal, and an anode disposed opposite the seed layer; and applying an electric current between the seed layer and the anode in such a manner as to satisfy the following formula (1), thereby electrolytically and electrochemically removing a passive layer formed on a surface of the seed layer: A≧1.15×B+5  (1) wherein “A” represents the sulfuric acid concentration of the electrolytic solution (g/L), and “B” represents current density (mA/cm²).
 11. The electrolytic processing method according to claim 10, wherein the seed layer is comprised of a ruthenium film or a ruthenium alloy film having a thickness of not more than 10 nm. 